The invention relates to a method for carrying out interference current or state monitoring by means of an interference current monitor or other state monitor in electrically operated rail vehicles which draw electrical energy from a power grid via a current collector and an overhead contact wire or a conductor rail and optionally feed it back into said grid in the braking mode.
Such rail vehicles require electrical energy at a high current and voltage level in order to be able to fulfill their driving function as well as numerous auxiliary functions. In a manner governed not only by the nonsteady-state operation (acceleration, braking) but also by the technology used (for example converters, but also other nonlinear elements such as transformers, etc.), the current consumption from the grid is necessarily not ideal, in other words always has interference components in addition to a DC component and/or fundamental-frequency AC component. A side effect of the high power level is that interference from the surroundings, including, in particular, from electrical equipment such as signal installations or telecommunications devices, becomes possible in the event of malfunctions in the vehicle. What is important for reliable and secure operation of railroad systems, therefore, is that interference that occurs is identified in good time and eliminated at a suitable time, depending on its severity. In this case, the range of possible measures extends from immediate protective disconnection, for example in the event of sudden failure of an important component and consequential massive exceeding of the interference current components which are permitted in respect of interference with signaling, through to simple database entry, the effect of which is that a part which wears slowly and is not safety-critical is replaced during the next periodic servicing.
FIG. 1 shows the basic concept of a simple interference current monitor, which is also referred to as interference monitor.
What is shown in this case is a diagrammatic illustration of a current path for the vehicle current from a railroad power grid (overhead contact line) 1 via a current collector 2, a main switch 3, driving and auxiliary facilities (traction equipment) 4, a vehicle wheel 5 to a rail 6. With the aid of one or more measuring elements 7, the primary current of the vehicle (vehicle current) is detected and processed in a monitor 8. If the monitor 8 records, in a process, that predetermined limits have been exceeded, it thereupon either intervenes directly in the traction equipment 4 or disconnects the vehicle from the overhead contact line 1 and thus from the electrical grid by means of the main switch 3.
Instead of an interference current monitor, it is also possible to use a so-called state monitor. Such a more general state monitor performs, in addition to the function of interference current monitoring ("interference monitoring"), which is a matter of protecting the entire railroad system and not the individual vehicle, also the function of protection for the vehicle ("vehicle protection"). For this protective function, critical internal (for example filter current) and external (for example grid voltage) quantities are measured and processed in order, if required, to disconnect the vehicle, or isolate it from the grid, for its own protection and for the protection of stationary installations (overhead contact line, substation).
The online monitoring of safety-critical measurement quantities in the case of rail vehicles, for instance of the grid current and its harmonic-frequency components with regard to the influencing of signal installations ("interference monitoring") and also for the protection of the vehicle ("vehicle protection"), is generally known.
In the case of the measured-value processing for this purpose, the use of analog or digital bandpass filters, of window functions in the time domain (for example rectangular windows, Hanning windows, etc.; these windows typically overlap), the discrete Fourier transform (=transformation by convolution with a reference signal of the desired frequency) and also the fast Fourier transform is likewise known.
FIG. 2 shows a known, generic basic structure of an interference current monitor with digital processing. In this case, a primary current I is detected as an analog measured value in a block 21. The measured value is digitized in a block 22--if appropriate after an analog preprocessing. The spectral density of interference currents is determined by means of digital processing in a block 23. In a block 24, finally, likewise by means of digital processing, the calculated spectral density is evaluated using a predetermined response threshold and an output A is formed which is forwarded to a control system or a main switch of the vehicle.
For the postprocessing of the spectral density values determined, in the block 24 either a comparatively long time window can be analyzed, with the result that even a single overshooting of a spectral limit value then effects a tripping of the monitor, or shorter windows can be analyzed and time conditions can be introduced for all the bins considered, for instance disconnection only after N consecutive repetitions or postprocessing with a single-ended delay element which directly counts up overshootings but, in the case of subsequent undershootings, resets the counter again only with a reduced speed.
The fundamental processing structure which is customary in this case is illustrated in FIG. 3. Proceeding from a sampled time signal x(t), values w(t) are formed by multiplication by window function and frequency bins X(f) are obtained by spectral transformation. These frequency bins are evaluated by comparison with a predetermined threshold value with regard to level and duration, for the purpose of forming the output signal A.
A disadvantage of the known interference monitoring methods illustrated in FIGS. 2 and 3 is that they are typically designed for steady-state operating conditions and that their behavior therefore frequently presents difficulties given the occurrence of transients or contact problems at the current collector (for example pantographs). If the monitoring, in other words the monitor, is set to be "highly alert", then the device trips in the event of many transients that occur, as a result of which the availability of the vehicle can be drastically reduced. If, on the other hand, said monitoring is set to be "more tolerant", typically by inserting empirical time conditions, then it is difficult to ensure the safety of the system. This conflict between safety and availability has resulted in a number of operators of railroad traffic systems pronouncing themselves completely against the use of interference current monitors, while others insist on monitors which additionally have to have a very high safety standard.
A further disadvantage of the known methods is the computational outlay required in the course of the digital processing in order to achieve a specific time and frequency resolution.